This invention relates generally to liquid-junction semiconductor devices for use as photocells and particularly to such devices for use as solar cells wherein both electrical and thermal energy is produced from light energy.
Concern over the continued availability as well as the continually escalating cost of fossil fuel energy sources has sustained high interest in the development of alternative energy sources, including solar power, which can be used to generate electricity. The devices most often considered for conversion of solar power into electricity are semiconductive devices, commonly called solar cells, which collect light, and generate photocurrent, in approximate proportion to the area of the photosensitive junction. This photosensitive junction must, therefore, be large to generate a useful current. The cost of manufacturing such devices depends in part on the area of the photosensitive junction and is presently too high to permit commercial exploitation of solar cells for other than limited and specialized applications.
The usual semiconductive devices for directly converting electromagnetic energy to electricity are photovoltaic cells "photocells," and common examples of photocells are silicon or gallium semiconductors having P-N junctions. Commonly, an electrical lead is connected on either side of the semiconductor across the P-N junction. Semiconductor photovoltaic cells are very expensive; in consequence, it has often been the practice to gather and concentrate the sunlight reaching a given semiconductor photocell so that extremely large areas of semiconductor material need not be employed as would be necessary without such a gathering system. The common gathering systems in the past were optical systems wherein lens systems concentrated light and focused the same on a given photovoltaic cell.
However, such a lens system was and is relatively expensive and is not useful in diffused light or on a cloudy day. More recently, however, there has been conceived a different type of collector and concentrator for radiation to be impinged on in a semiconductor photocell. For instance, Weber and Lambe, in Applied Optics, Vol. 15, pgs. 2299-2300, October, 1976, disclose a system whereby a large area sheet of material, such as a rigid plastic or a glass doped with luminescent material is exposed to solar radiation. The luminescent material ideally has a strong absorption of the sun's rays, especially in the visible region where the solar spectrum peaks, and it emits electromagnetic radiation of a longer wavelength suitable for activating the semiconductor photocell. A large portion of the light emitted from the luminescent material is in effect trapped in the collector with essentially total internal reflection until the light reaches an area where a photocell, such as a silicon photocell, is optically coupled to a small area, for instance, an edge of the collector. In this way, the light from the sun is not only converted to more suitable wavelengths for activation of the photocell but is concentrated, since the light received by the large area of the collector escapes only in the small area where the photocell is optically connected to the collector.
Another article by Levitt and Weber, appearing in Applied Optics, Vol. 16, 10, pgs. 2684-2689, October, 1977, should be read with the article first mentioned. Other publications aiding in the understanding of luminescent solar collectors include Geotzberger, Applied Physics, 14, 123-139 (1977) and U.S. Pat. No. 4,110,123 issued August, 1978, claiming priority in part based on German Patent Application Nos. 2620115 published Nov. 10, 1977, filed May 6, 1976, and 2628917 published Jan. 12, 1978, filed June 24, 1976, and referred to in the former patent application. German Patent Application No. 2554226, published June 6, 1977, is of some peripheral interest.
Also, numerous patents deal with the conversion of solar energy to different wavelengths by means of luminescent or fluorescent layers and impinging emitted light on a photocell; examples are U.S. Pat. Nos. 3,426,212, 3,484,606 and 3,912,931. In U.S. Pat. No. 3,912,931, benzene and other aromatic hydrocarbons are said to be "fixed" in layers of a silicone resin superimposed on a photocell.
U.S. Pat. No. 4,186,033, issued Jan. 29, 1980, to Boling et al, discloses a structure for a conversion of solar energy to electricity and heat using a luminescent solar collector and concentrators in conjunction with a photovoltaic cell. The device of the above-mentioned reference works in the same general way as the devices disclosed in the Goetzberger et al. U.S. Pat. No. 4,110,123 and German Application No. 2620115, in the Weber and Lambe paper, and in the Levitt and Weber paper, but in addition claims an improved structure.
U.S. Pat. No. 4,081,289 shows a scheme for cooling solar cells as they generate electricity but in a very different setting from the above-mentioned patents and articles.
U.S. Pat. No. 4,056,405 issued Nov. 1, 1977, to Varadi discloses a solar panel structure which accepts a plurality of individual solar cells and provides a method of cooling these solar cells. Varadi is concerned with providing a housing to protect solar cells from both the environment and from overheating. Varadi does recognize that the heat removed may be utilized and is not just a waste product.
U.S. Pat. No. 4,172,740 issued Oct. 30, 1979, to Campbell discloses a solar cell and light concentration system wherein a liquid contained in a sphere is used as a lens system for solar cells located substantially in the center of said sphere. A method of extracting heat from the fluid is also disclosed.
None of these references, however, teach the combination of thermal and electrical production utilizing a photoelectrochemical cell in conjunction with a thermal transfer medium. Additionally, none of these references disclose a system wherein materials are used which are not highly toxic to the environment, i.e., they all discuss the use of the commonly used photovoltaic cells containing elements such as gallium, arsenic, selenium, etc., or disclose a heat transfer system with only generalizations of solar cells without discription of any specific solar cells.
While photoelectrochemical cells (PEC) offer an alternative to photovoltaic cells, they too have had the problem of being expensive to produce. Considerable effort has been devoted to finding ways to reduce the cost of semiconductor solar cell devices. Must of this effort has been directed, as in U.S. Pat. No. 3,953,876 issued Apr. 27, 1976, to devices in which the semiconductor material is desposited as a polycrystalline thin film on an inexpensive substrate rather than grown by the costly single crystal techniques used in earlier solar cells. A different approach that has generated enthusiasm is the use of a liquid-junction semiconductor solar cell. The active part of these cells is a junction formed at a semiconductor-liquid interface. Because the junction forms spontaneously at the liquid-solid interface, the device promises to be less costly to manufacture because relatively costly epitaxy or diffusion procedures required for the single crystal or polycrystalline devices mentioned above are not needed to form the junction.
However, four obstacles must be surmounted before such photocells can be exploited commercially. First, liquid-junction semiconductors ofter are not photochemically stable because the photoexcitation produces excess minority charge carriers at the semiconductor surface which may react with the semiconducting material, causing corrosion of the semiconductor surface. This corrosion proceeds in a manner that degrades the desired characteristics of the semiconductor surface and is manifested by the decay of the photocurrent from the cell with operating time. An example of such a reaction with a CdS electrode, for example, is CdS(s)+2h.sup.+ .fwdarw.S.sup.0 (s)+Cd.sup.++ (solvated), leading to the formation of a sulfur layer at the junction interface. One approach to solving this problem involves the use of, for example, a polysulfide-sulfide redox couple type of solution. Since the corrosion reaction CdS(s)+2.sup.+ .fwdarw.Cd.sup.++ (solvated)+S(s) proceeds at a higher electrode potential then the reaction S.sup.-- .fwdarw.S+2e, the sulfur-polysulfide couple consumes the holes responsible for the corrosion reaction before the potential for the corrosion reaction is reached. A second approach to resolving this problem is to use a material which has a corrosion reaction potential so high as to be in effect corrosion resistant. Such materials are, for example, certain transition metal oxide compounds. A specific example is titanium dioxide.
Secondary, the cost of single crystal semiconductor electrodes is too high for commercial success. Several approaches have been tried to reduce the cost of single crystal semiconductors, especially chalcogenide electrodes. One approach involves the electrolytic co-deposition of the electrode materials, e.g., cadmium and selenium, on an inert substrate. Another approach involves the anodization of a cadmium or bismuth substrate to form a chalcogenide semiconductor. These methods, however, do not produce materials which are cost competitive in the market place.
Thirdly, the band gap of the photoelectrodes must be closely attuned to the major energy portion of the solar spectrum, i.e., approximately 1.4 eV. This band gap is necessary not only to produce greater power per surface area, thereby increasing the output of a given cell, but also to decrease the area of the liquid-solid junction needed and thereby lower the cost of the installation per unit of energy produced.
Finally, the liquid-junction semiconductor photocell needs to be one which is environmentally sound. Thus, while material such as cadmium and selenium may produce potentially useful power outputs when used in solar cells, they are themselves highly toxic materials. Therefore, they are not only environmentally harmful in use but also difficult and expensive to manufacture due to the necessary environmental considerations needed in the manufacturing processes of these materials.